Confined rapid thermolysis studies of ammonia borane

Thermochemical calculations of ammonia borane (AB, H3NBH3), which has a hydrogen content of 19.6% by weight, indicate that it has the potential to boost specific impulse in chemical propulsion applications due to its high hydrogen content and the moderate exothermicity of decomposition. Research to date on AB decomposition has focused on relatively slow heating rates. These studies have shown that the mass lost due to decomposition increases with increasing heating rate. This trend has been confirmed in this work, as mass loss continues to increase up to 50 K/min, the limit of most TGA/DSC instruments. In this research effort, confined rapid thermolysis was used to examine the decomposition of AB under isothermal conditions. Fourier transform infrared (FTIR) spectroscopy and time-of-flight mass spectrometry (ToF-MS) were employed to identify the gaseous products, which include H2, NH3, H2NBH2, and c-N3B3H6. The decomposition resulted in significant condensed-phase products as well, which were pressed into a KBr pellet and examined with FTIR spectroscopy. FTIR transmission spectra of the condensed-phase products with several heating durations show the disappearance of absorption bands of AB and appearance of bands attributed to polymeric species. Condensable gas-phase products were also collected from the stream of decomposition products, and FTIR spectroscopy showed they have absorption bands similar to the polymeric species, indicating that the H2NBH2 will readily condense out of the gas-phase products and polymerize at low temperatures.

► Ammonia borane (NH3BH3) was subjected to high external heating rates, to better simulate the conditions in a propellant.
► Gas-phase decomposition products were measured using FTIR spectroscopy and ToF-MS.
► Gaseous decomposition products contained H2, NH3, H2NBH2, and at later times, borazine (c-N3B3H6).
► Condensed-phase products were examined using FTIR spectroscopy and showed polymerization.